Stacked FET Switch Bias Ladders

ABSTRACT

A positive-logic FET switch stack that does not require a negative bias voltage, exhibits high isolation and low insertion/mismatch loss, and may withstand high RF voltages. Embodiments include a FET stack comprising series-coupled positive-logic FETs (i.e., FETs not requiring a negative voltage supply to turn OFF), series-coupled on at least one end by an “end-cap” FET of a type that turns OFF when its VGS is zero volts. The one or more end-cap FETs provide a selectable capacitive DC blocking function or a resistive signal path. Embodiments include a stack of FETs of only the zero VGS type, or a mix of positive-logic and zero VGS type FETs with end-cap FETs of the zero VGS type. Some embodiments withstand high RF voltages by including combinations of series or parallel coupled resistor ladders for the FET gate resistors, drain-source resistors, body charge control resistors, and one or more AC coupling modules.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application may be related to the following patents andpatent applications, the contents of all of which are incorporatedherein by reference:

-   -   U.S. Pat. No. 8,669,804, issued on Mar. 11, 2014 and entitled        “Device and Methods for Improving Voltage Handling and/or        Bi-directionality of Stacks of Elements when Connected Between        Terminals”;    -   U.S. patent application Ser. No. 15/256,453, filed Sep. 2, 2016        and entitled “Positive Logic Digitally Tunable Capacitor”;    -   U.S. patent application Ser. No. 15/289,768, filed Oct. 10,        2016, entitled “Reduced Dissipation Switch FET Gate Biasing”,        which claims priority to U.S. Provisional Application No.        62/241,498, filed Oct. 14, 2015;    -   U.S. patent application Ser. No. ______, filed concurrently        herewith, Attorney Docket No. PER-245-PAP, entitled “Positive        Logic Switch with Selectable DC Blocking Circuit”;    -   U.S. patent application Ser. No. ______, filed concurrently        herewith, Attorney Docket No. PER-257-PAP, entitled “AC Coupling        Modules for Bias Ladders”.

BACKGROUND (1) Technical Field

The invention relates to electronic circuits, and more particularly toradio frequency electronic switch circuits and related methods.

(2) Background

In radio frequency (RF) systems, such as cellular phones and WiFinetworks, electronic switch circuits are often used in series with RFsignal lines to selectively block or conduct RF signals, such as betweenan antenna and a transceiver circuit. Electronic switch circuits arealso often used in a shunt configuration between an RF signal line and areference potential (e.g., circuit ground), for example, to selectivelyisolate nodes of the RF signal line from significantly influencing othercircuitry. As one example, FIG. 1 is a simplified schematic circuit of acommon prior art series-shunt switch circuit configuration 100. In theillustrated configuration, a series switch circuit 102 is coupled inseries with an RF signal line 104 between an RF_(IN) port and anRF_(OUT) port, and a shunt switch circuit 106 is coupled to the RFsignal line 104 between the RF_(OUT) port and circuit ground. Asillustrated, the series switch circuit 102 and shunt switch circuit 104are modeled as single-pole, single-throw (SPST) switches. In use, theseries switch circuit 102 and shunt switch circuit 106 are generallyoperated in a complementary manner: if series switch circuit 102 isopen, then shunt switch circuit 106 is closed, and if series switchcircuit 102 is closed, then shunt switch circuit 106 is open.

Ideally, switch circuits such as those shown in FIG. 1 should notappreciably alter or affect an RF signal. However, in integratedcircuits, RF switching circuits are generally implemented withtransistors, particularly field-effect transistors (FETs), and moreparticularly MOSFETs. A FET in a conducting (ON) state presents someresistance, R_(ON), to a conducted signal, and in a blocking (OFF) statepresents some capacitance, C_(OFF), which may be in series, shunt, orparallel to an RF signal line. Accordingly, FET-based switch circuitsgenerally behave less than ideally in an RF circuit.

A further issue with FETs is that the voltage that a single FET canwithstand between drain and source without breaking down is generallylimited to a few volts. In an electronic system, there may be parts ofthe system where the voltage that must be withstood far exceeds thevoltage handling capability of a single FET. A common solution is toseries stack FETs so that the drain-source voltage across any one FET isless than its drain-source breakdown voltage.

For example, FIG. 2 is a schematic circuit of a prior art series-shuntRF switch circuit 200 using FET stacks. In this example, an RF signalfrom an antenna ANT is coupled from RF_(IN) through a series stack 202to RF_(OUT). A shunt stack 204 is coupled between RF_(OUT) and circuitground. Each of the stacks 202, 204 comprises a plurality ofseries-coupled FETs M1-Mn (although “n” may differ for the two stacks202, 204). In this example, each FET includes a dedicated gate resistorRg.

A standard biasing scheme is usually used in this type of circuit 200when configured with FET switches having a common threshold voltage andchannel structure. A positive voltage is applied to the gate of each FETM1-Mn of a stack 202, 204 to turn the stack ON (conducting), and anegative voltage is applied to the gate of each FET M1-Mn of a stack202, 204 to turn the stack OFF (blocking). By way of example, a positiveDC voltage of +3 volts may be applied (through the resistors Rg) to thegates of each FET M1-Mn of a stack 202, 204 to turn the stack ON, and anegative DC voltage of −3 volts may be applied (again through theresistors Rg) to the gates of each M1-Mn of a stack 202, 204 to turn thestack OFF. For good performance, it is desirable to use a large enoughnegative voltage to fully turn each FET switch M1-Mn OFF. Turning theFET switches into a full OFF state results in improved RF linearity interms of harmonics and inter-modulation distortion (IMD), better RFpower handling, higher isolation, and lower leakage current, among otheradvantages.

In most applications using an integrated circuit (IC) implementation ofthe circuit shown in FIG. 2, a positive supply voltage is readilyavailable from an external source. A negative supply voltage is usuallygenerated on-chip using a bias generator 206, shown in FIG. 2 as coupledto the gate resistors Rg. The bias generator 206 may conventionally useone or more negative voltage charge pumps and voltage regulators thatconvert an externally supplied positive supply voltage to a negativevoltage.

The configuration shown in FIG. 2 works reasonably well for manyapplications. However, there are applications where generating anegative supply voltage is either not desired or not practical due todesign constraints; in some applications, use of a negative charge pumpis not even possible. These are generally applications with stringentrequirements such as extremely low current and power consumption,extremely low noise sensitivity, and/or very small IC die areas.Moreover, in biasing schemes wherein charge pumps are used to generatenegative supply power, switching speed is limited by the currentsourcing capability of the charge pumps. Further, the voltage swing onthe FET gates from full ON to full OFF is significant—from +3V to −3V inthe example above, or a 6V swing, which further limits switching speed.In addition, the charge swing has to go through the series gateresistors of the FETs, which dampens and slows the switching process.

Solutions to utilizing a FET switch stack without using a negativesupply voltage are taught in co-pending U.S. patent application Ser. No.15/256,453, filed Sep. 2, 2016 and entitled “Positive Logic DigitallyTunable Capacitor”. FIG. 3 is a schematic circuit of a series-shunt RFswitch circuit 300 using “positive logic” FET stacks. In accordance withone embodiment of the referenced patent application, an RF signal froman antenna ANT is coupled from RF_(IN) through a series stack 302 toRF_(OUT). A shunt stack 304 is coupled between RF_(OUT) and circuitground. Each of the stacks 302, 304 comprise a plurality ofseries-coupled FETs M1-Mn (although “n” may differ for the two stacks302, 304) which have the characteristic that each FET turns OFFsufficiently when Vgs≈0V. In this example, each FET includes a dedicatedgate resistor Rg. Each stack 302, 304 also includes two bracketing DCblocking capacitors C connected in series with the FETs M1-Mn; theblocking capacitors C may each have different values for particularapplications.

Each of the stacks 302, 304 is configured to be coupled to twonon-negative supply voltages, Vgate and Vdrain, which are used tocontrol the switch state, ON or OFF, of associated FETs M1-Mn. The Vgatesupply voltage is coupled to the gates of the FETs M1-Mn of each stack302, 304 through corresponding parallel gate resistors Rg. The Vdrainsupply voltage is coupled to a network of parallel resistors Rds thatare coupled as shown to respective drains or sources of each of the FETsM1-Mn, including shared drains-source nodes between the FETs M1-Mn.

In the illustrated example, the FETs M1-Mn are turned ON by applying apositive Vgate voltage (e.g., +3V) to the FET gates through theirrespective gate resistors Rg, while applying a Vdrain voltage of 0V tothe FET drains and sources through respective Rds resistors. Thisconfiguration effectively creates a positive DC gate-source voltage,V_(GS), for every FET M1-Mn (i.e., V_(GS)=+3V with respect to the sourcevoltage of 0V if Vgate=+3V). The FET stacks are turned OFF by applying aVgate voltage of 0V to the FET gates through their respective gateresistors Rg, while applying a positive Vdrain voltage (e.g., +3V) tothe FET drains and sources through respective Rds resistors. Thiseffectively creates, in relative terms, a negative V_(GS) for every FETM1-Mn (i.e., V_(GS)=−3V with respect to the source voltage of +3V ifVgate=0V).

Obtaining a negative V_(GS) for the OFF state is accomplished by virtueof the DC blocking capacitors C on both ends of each stack 302, 304 toachieve the required gate-source voltage levels in both the ON and OFFstates. Without the blocking capacitors C, the “end” FETs (i.e., M1, Mn)would have either a drain or source DC coupled to some potential (e.g.,circuit ground) that would not allow the required V_(GS) voltage to beapplied to the FETs M1-Mn. With the DC blocking capacitors C, when apositive Vdrain voltage is applied, a positive voltage can be built upon the drain and source nodes of the FETs M1-Mn as the capacitors Ccharge. Other variations and modes of operation are also disclosed inU.S. patent application Ser. No. 15/256,453.

Circuits of the type shown in FIG. 3 avoid use of a negative voltage andthus require no negative voltage charge pump. Switching speed isincreased because discharge of the FET gates does not have to go througha charge pump. Accordingly, operation without a charge-pump enables azero current standby mode, with very fast switching and settling timesand a valid RF state in standby mode, as well as other advantages asdisclosed in the U.S. patent application Ser. No. 15/256,453. However,there is room for improvement.

In particular, the DC blocking capacitors C have to be large for highisolation and for low insertion loss and low mismatch loss (i.e., theirimpedance at a desired operating frequency needs to be low), thusconsuming IC die area and increasing cost. The constant presence of theDC blocking capacitors C in both ON and OFF states for a series stack302 affects a conveyed RF signal, while their constant presence and sizeimpacts isolation. Further, the constantly present DC blockingcapacitors C have to be charged through the Rds resistor network, whichimpacts switching and settling times (this is in addition to the usualRg*Cgs time constant on the gate terminal side). Moreover, the DCblocking capacitors C are generally formed as metal-insulator-metal(MIM) capacitor structures, which are sensitive to electro-staticdischarge (ESD) and thus require ESD protection on RF ports (e.g.,RF_(IN)).

In addition, in practical applications, a series stack 302 generallyrequires an integrated shunt tuning inductor L coupled near each DCblocking capacitor C to perform both impedance matching (i.e.,resonating out the capacitance of the DC blocking capacitors C) and ESDprotection for port connections (by providing a DC path to a referencepotential). Such added shunt tuning inductors L, in combination with theadjacent blocking capacitors C, cause the series stack 302 to have ahigh-pass filter response, affecting the RF performance of the switch.The shunt tuning inductors L also consume IC area, thus increasing cost.

A further area for improvement is in handling high voltage. Embodimentsof the type shown in FIG. 3 may breakdown when subjected to a highvoltage RF signal due to RF voltages being coupled through to gateresistors, causing breakdown.

Accordingly, there is a need for an improved FET switch stack that doesnot require a negative bias voltage, but which also avoids or mitigatesthe drawbacks of positive logic FET stacks as described above, and whichcan withstand, in some embodiments, application of a high voltage RFsignal. The present invention addresses these and other needs.

SUMMARY

The invention encompasses an improved positive-logic FET switch stackthat does not require a negative bias voltage, exhibits high isolationand low insertion loss and mismatch loss characteristics, requires lessintegrated circuit (IC) area, and in some embodiments, can withstandapplication of a high voltage RF signal.

Embodiments include a FET stack comprising one or more series-coupledpositive-logic FETs M1-Mn (i.e., zero-Vt, low-Vt, or high-Vt FETs of atype that require a negative V_(GS) to turn OFF but configured so as tonot require a negative power supply), series-coupled on at least one endto an “end-cap” FET M₀, where “M₀” designates a FET of a high-Vt typethat turns OFF when the V_(GS) of such FET is essentially zero volts.The end-cap M₀ FETs selectably provide either a capacitive DC blockingfunction or a resistive signal path. Some embodiments may comprise astack of only M₀ FETs, or a mix of positive-logic FETs and M₀ FETs, solong as at least one end-cap FET is an M₀ FET. Optional end-capcapacitors may be included to prevent early breakdown of correspondingend-cap M₀ FETs.

Some embodiments are configured to withstand high applied RF voltages byincluding a series-connected bias resistor ladder for the gates of theFETs in the stack (i.e., M1-Mn, M₀), a series-connected Rds drain-sourceresistor ladder, and either a series-connected or a parallel-connectedRb body charge control resistor ladder. Some embodiments may useseries-connected Rb body charge control resistor ladder in conjunctionwith a parallel-connected bias resistor ladder for the gates. Someembodiments may use a series-connected bias resistor ladder (in a “rail”configuration) in conjunction with a parallel-connected bias resistorladder (in a “rung” configuration) for the gates.

Some embodiments are configured with one or more AC coupling moduleseach coupled to at least one end of a corresponding gate bias resistorladder and/or body charge control resistor ladder, and configured to becoupled to a radio frequency voltage source.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic circuit of a common prior artseries-shunt switch circuit configuration.

FIG. 2 is a schematic circuit of a prior art series-shunt RF switchcircuit using FET stacks.

FIG. 3 is a schematic circuit of a series-shunt RF switch circuit using“positive logic” FET stacks.

FIG. 4 is a schematic circuit of an improved positive logic FET stack.

FIG. 5A is a table showing example voltages for Vgate and Vdraincorresponding to ON and OFF states for the FET stack of FIG. 4.

FIG. 5B is a simplified schematic circuit of the FET stack of FIG. 4showing example drain and source voltages for the interior FETs Mn andend-cap FETs M₀ when in an ON state.

FIG. 5C is a simplified schematic circuit of the FET stack of FIG. 4showing example drain and source voltages for the interior FETs Mn andend-cap FETs M₀ when in an OFF state.

FIG. 6 is a schematic diagram of a series-shunt RF switch circuit usingthe improved positive logic FET stacks of FIG. 4.

FIG. 7 depicts a schematic equivalent circuit of a portion of a priorart FET stack, showing parasitic capacitances.

FIG. 8A is a schematic equivalent circuit of a FET stack with aseries-connected resistor ladder gate bias scheme in accordance with oneaspect of the present invention, showing parasitic capacitances.

FIG. 8B is a schematic diagram of a positive logic FET stack with aseries-connected gate bias resistor ladder and a series-connecteddrain-source resistor ladder in accordance with the present invention.

FIG. 8C is a schematic diagram of a positive logic FET stack with anadded series-connected resistor ladder for controlling body charge.

FIG. 8D is a schematic diagram of a conventional FET stack with aseries-connected gate bias resistor ladder, a series-connecteddrain-source resistor ladder, and a series-connected body charge controlresistor ladder.

FIG. 8E is a schematic diagram of a conventional FET stack with aseries-connected gate bias resistor ladder, a series-connecteddrain-source resistor ladder, and a parallel-connected body chargecontrol resistor ladder.

FIG. 8F is a schematic diagram of a conventional FET stack with aseries-connected gate bias resistor ladder and an AC Coupling GateModule.

FIG. 8G is a schematic diagram of a conventional FET stack with aseries-connected gate bias resistor ladder and dual AC Coupling GateModules.

FIG. 8H is a simplified schematic diagram of an SOI MOSFET that includesan ACS structure along with conventional drain D, source S, and gate Gterminals and a gate resistor Rg.

FIG. 9 is a process flow diagram showing one method for providing a FETswitch stack.

FIG. 10 is a process flow diagram showing one method for providing aseries-shunt switch circuit.

FIG. 11 is a process flow diagram showing one method for biasing a FETswitch stack.

FIG. 12 is a process flow diagram showing a method for providing a FETswitch stack.

FIG. 13 is a process flow diagram showing another method for biasing aFET switch stack.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The invention encompasses an improved positive-logic FET switch stackthat does not require a negative bias voltage, exhibits high isolationand low insertion loss and mismatch loss characteristics, requires lessintegrated circuit (IC) area, and in some embodiments, can withstandapplication of a high voltage RF signal.

Embodiments include a FET stack comprising one or more series-coupledpositive-logic FETs M1-Mn (i.e., zero-Vt, low-Vt, or high-Vt FETs of atype that require a negative V_(GS) to turn OFF but configured so as tonot require a negative power supply), series-coupled on at least one endto an “end-cap” FET M₀, where “M₀” designates a FET of a high-Vt typethat turns OFF when the V_(GS) of such FET is essentially zero volts.The end-cap M₀ FETs selectably provide either a capacitive DC blockingfunction or a resistive signal path. Some embodiments may comprise astack of only M₀ FETs, or a mix of positive-logic FETs and M₀ FETs, solong as at least one end-cap FET is an M₀ FET. Optional end-capcapacitors may be included to prevent early breakdown of correspondingend-cap M₀ FETs.

Some embodiments are configured to withstand high applied RF voltages byincluding a series-connected bias resistor ladder for the gates of theFETs in the stack (i.e., M1-Mn, M₀), a series-connected Rds drain-sourceresistor ladder, and either a series-connected or a parallel-connectedRb body charge control resistor ladder. Some embodiments may useseries-connected Rb body charge control resistor ladder in conjunctionwith a parallel-connected bias resistor ladder for the gates. Someembodiments may use a series-connected bias resistor ladder (in a “rail”configuration) in conjunction with a parallel-connected bias resistorladder (in a “rung” configuration) for the gates.

Some embodiments are configured with one or more AC coupling moduleseach coupled to at least one end of a corresponding gate bias resistorladder and/or body charge control resistor ladder, and configured to becoupled to a radio frequency voltage source.

Embodiments of the invention may be used in any circuit configuration inwhich a FET stack may be necessary or useful, such as RF switch circuitsand digitally tuned capacitors (for example, of the type taught in U.S.patent application Ser. No. 15/256,453).

Selectable DC Blocking Embodiments

FIG. 4 is a schematic circuit of an improved positive logic FET stack400. The stack 400 comprises one or more series-coupled FETs M1-Mn. Eachof the FETs M1-Mn are of a type that require a negative V_(GS) to turnOFF (e.g., −3V) but which are configured to not require a negative powersupply, as taught in co-pending U.S. patent application Ser. No.15/256,453. Examples of such FETs M1-Mn may have a V_(T)≈0V and a lowR_(ON) (e.g., much less than Z₀ ohms, where Z₀ is the systemcharacteristic impedance).

In some embodiments, the FETs M1-Mn in the “interior” of the stack 400,which may be zero-Vt, low-Vt, or high-Vt FETs, are bracketed by“end-cap” M₀ high-Vt FETs. In other embodiments, only one end of one ormore series-coupled positive-logic FETs M1-Mn is series coupled to an M₀FET; thus, for example, the end-cap M₀ FET in dashed box 402 or theend-cap M₀ FET in dashed box 404 may be omitted in some applications.Zero-Vt, low-Vt and high-Vt FETs are available from a number ofdifferent IC commercial foundries. Examples of such end-cap M₀ FETs mayhave a V_(T)≈0.7V, and may have a higher R_(ON) than the FETs M1-Mn.

Each FET M1-Mn, M₀ includes an associated gate resistor Rg; in theillustrated example, the gate resistors Rg are dedicated to each FET andparallel to each other (for other configurations, see below).

Two non-negative supply voltages, Vgate and Vdrain, are used to controlthe switch state, ON or OFF, of associated FETs M1-Mn, M₀. The Vgatesupply voltage is coupled to the gates of the FETs of the stack 400through corresponding dedicated parallel gate resistors Rg. The Vdrainsupply voltage is coupled to a network of parallel resistors Rds thatare coupled as shown to respective sources or drains of each of the FETsM1-Mn, including shared drains-source nodes between the FETs M1-Mn. Inthe embodiment of FIG. 4, the gate resistors Rg and the networkresistors Rds may be of equal size, but a person skilled in the art willunderstand that other embodiments may include resistors of differentsizes.

In the embodiment shown in FIG. 4, the drain and source of each end-capM₀ FET is coupled in parallel with a capacitor Cd. As disclosed ingreater detail below, since the end-cap M₀ FETs are biased with Vgs=0Vand thus usually cannot handle the full drain-source voltage Vds appliedacross them, the Vds across each end-cap M₀ FET is lowered (e.g., to 2V)by their respective capacitor Cd.

FIG. 5A is a table showing example voltages for Vgate and Vdraincorresponding to ON and OFF states for the FET stack of FIG. 4. FIG. 5Bis a simplified schematic circuit of the FET stack of FIG. 4 showingexample drain and source voltages for the interior FETs Mn and end-capM₀ FETs when in an ON state. FIG. 5C is a simplified schematic circuitof the FET stack of FIG. 4 showing example drain and source voltages forthe interior FETs Mn and end-cap M₀ FETs when in an OFF state.

In the example illustrated in FIGS. 4 and 5B, the FETs FET M1-Mn, M₀ areturned ON by applying a positive Vgate voltage (e.g., +3V) to the FETgates through their respective gate resistors Rg, while applying aVdrain voltage of 0V to the drains and sources of FETs M1-Mn throughrespective Rds resistors. (Note that, technically, the source for the“top” M₀ FET would be, for example, a coupled antenna or other RF signalsource, and thus its drain is at the “bottom” end of that FET.Similarly, the source for the “bottom” M₀ FET would be, for example,circuit ground, and thus its drain is at the “top” end of that FET.Accordingly, the Vdrain voltage is coupled to the respective drains ofboth the “top” M₀ FET and the “bottom” M₀ FET. As should be clear, “top”and “bottom” are with respect to the illustrated orientation of the FETstack in FIG. 4 and are for convenience only). This configurationeffectively creates a positive DC gate-source voltage, V_(GS), for everyFET M1-Mn, M₀ (i.e., V_(GS)=+3V with respect to the source voltage of 0Vif Vgate=+3V), as shown in FIG. 5B. In the ON state, the end-cap M₀ FETseach present a series resistance of R_(ON). Since the ON-state end-capM₀ FETs essentially present no series capacitance, shunt tuninginductors L for impedance matching may be omitted, thus improvingbroadband performance and reducing consumption of IC area.

In the example illustrated in FIGS. 4 and 5C, the FETs FET M1-Mn, M₀ areturned OFF by applying a Vgate voltage of 0V to the FET gates throughtheir respective gate resistors Rg, while applying a positive Vdrainvoltage (e.g., +3V) to the drains and sources of FETs M1-Mn (and thus tothe source of the “top” M₀ FET, and to the drain of the “bottom” M₀ FET)through respective Rds resistors. This effectively creates a negativeV_(GS) for every FET M1-Mn (i.e., V_(GS)=−3V with respect to the sourcevoltage of +3V if Vgate=0V), as shown in FIG. 5C. The end-cap M₀ FETshave either a V_(GS) of −3V (for the “top” M₀ FET) or 0V (for the“bottom” M₀ FET), either of which is sufficient to turn the end-cap M₀FETs OFF in light of their V_(GS) characteristics (although the currentblocking capability of the end-cap M₀ FETs increases as V_(GS) becomesmore negative). In the OFF state, the end-cap M₀ FETs each present aseries capacitance of C_(OFF), and thus perform a DC blocking function.

As an alternative embodiment, when the “bottom” M₀ FET is ON, theterminal labeled Vdrain in FIG. 4 can be left as an open circuit, sinceall of the FETs coupled to the Rds resistor network would be coupled tocircuit ground through the “bottom” M₀ FET. For the OFF state, theterminal labeled Vdrain in FIG. 4 would be coupled to a suitable voltage(e.g., +3V).

In the example embodiment of FIG. 4, Vdrain is varied between 0V to 3Vas a function of the ON or OFF state of the FET stack 400. In otherembodiments, Vdrain may be set as a fixed voltage, such as a constantmid-rail voltage (e.g., +1.5V). Thus, similar to the example shown inFIG. 5A-5C, when Vgate is high (e.g., +3V), a positive voltage of +1.5Vwill appear across the gate-source of each FET M1-Mn, M₀ and turn thoseFETs ON. Conversely, when Vgate is low (e.g., 0V), a negative voltage of−1.5V will appear across the gate-source of each interior FET M1-Mn andturn those FETs OFF; similarly, V_(GS) for the “top” end-cap M₀ FET willbe −1.5V, while the V_(GS) for the “bottom” end-cap M₀ FET will be 0V,and thus both of the end-cap M₀ FETs will be OFF. In a constant Vdrainbiasing scheme, lower or higher voltages (e.g., +0.5V) may also be usedfor Vdrain. In a further embodiment, the fixed mid-rail voltage may bechosen based on the threshold voltage, V_(T), of the FETs to provide abeneficial (potentially optimal) level of gate overdrive (V_(GS)−V_(T))to balance performance parameters in both ON and OFF states. Moregenerally, the voltages for Vgate and Vdrain can be selected (andpotentially optimized) to provide a balanced amount of gate overdrive.If Vdrain is biased to a non-zero DC voltage, a correctly sized shuntcapacitor (or RC filter) may need to be added to sufficiently filter outRF voltage swings.

The circuit configuration of the FET stack 400 of FIG. 4 is thus similarto the circuit configurations of the stacks 302, 304 of FIG. 3, exceptthat the end-cap M₀ FETs are used for DC blocking instead of explicit DCblocking capacitors C. In operation, the end-cap M₀ FETs can be turnedto an OFF state in which they function as DC blocking capacitors havinga capacitance of C_(OFF). Importantly, the end-cap M₀ FETs also can beturned to an ON state that forms an actual resistive DC path rather thana capacitance, resulting in little or no insertion loss, mismatch loss,or frequency dependence penalty compared to constantly present DCblocking capacitors C, and also decreasing switching and settling times.While the R_(ON) for the end-cap M₀ FETs generally may be greater thanthe R_(ON) for the interior FETs M1-Mn, with larger numbers of interiorFETs M1-Mn (i.e., higher stack heights), the R_(ON) of the interior FETsM1-Mn will dominate the series resistance.

A further benefit of using one or more end-cap M₀ FETs as DC blockingcapacitances is that they provide automatic ESD protection, because theFET stack is self-protected by drain-to-source breakdown and there areno additional series components between the FET drain-source terminalsand the external ports (ESD pulses are applied across external ports).Accordingly, shunt tuning inductors L for ESD protection of MIMcapacitors in FET stacks coupled to ports may be omitted, thus reducingconsumption of IC area. In addition, using one or more end-cap M₀ FETsprovides a reasonable “un-powered OFF” state for applications thatbenefit from some isolation when the system voltage supply, V_(CC),equals 0V.

In some embodiments, all of the FETs in a stack 400 may be of the M₀type. In other embodiments, some (but not all) of the interior FETs in astack 400 may be of the M₀ type, so long as at least one end-cap M₀ FETis provided. For example, the stack 400 may comprise a mix of FETs in asequence such as M₀-M1-M₀-M2-M₀, where M1 and M2 are positive-logicFETS. Such mixed configurations may have a higher series resistance thanthe embodiment shown in FIG. 4, due to higher R_(ON) for the M₀ FETs,but would have a lower R_(ON) than a stack comprising all M₀-type FETs.Mixed configurations of FET types and sequences can be used to helpoptimize the total stack performance parameters of R_(ON), OFFisolation, peak voltage (Vpk) handling, DC operation, and un-poweredcircuit behavior.

Referring back to FIG. 4, optional capacitors Cd may be coupled inparallel with the end-cap M₀ FETs, from drain to source, as indicated bythe dotted connector lines. When the end-cap M₀ FETs are in an ON state,the optional capacitors Cd are effectively bypassed and thusfunctionally out of the RF signal path. When the end-cap M₀ FETs are inan OFF state, the optional capacitors Cd are coupled to the RF signalpath in parallel with the end-cap M₀ FETs (which each present as acapacitance of C_(OFF)). One advantage of the optional capacitors Cd isthat they reduce RF voltage swing across the end-cap M₀ FETs (e.g., to2V instead of a full 3V) and thus help prevent early breakdown of thoseFETs. It may be desirable for the optional capacitors Cd to have acapacitance that is about 5-10 times the C_(OFF) of the end-cap M₀ FETs.In some embodiments, only the “bottom” M₀ FET (i.e., nearest circuitground in FIG. 4) includes the capacitor Cd.

With further reference to FIG. 4, the person skilled in the art willappreciate that, regardless of the switching state of the FET stack 400,and except for leakage currents, the FET stack 400 and generation of thebias voltages Vgate and Vdrain does not consume any significant DCpower, unlike the case of FET stacks that require a charge pump. This isvery beneficial for integrated circuits with stringent power or currentconsumption requirements. Embodiments may use the existing systemvoltage supply rail to provide Vgate and Vdrain; however, a personskilled in the art will understand that other design choices to provideVgate and Vdrain may be envisaged.

In yet other embodiments, the gates of one or both of the end-cap M₀FETs may be biased separately from the interior FETs M1-Mn. For example,biasing the gates of both end-cap M₀ FETs with a negative gate voltage(and thus a negative V_(GS)) would more completely turn both FETs OFFand improve the linearity and harmonics performance of those FETs, butwould not require a significant current draw since such FETs inherentlydraw low body current and are only two in number. Negative biasing wouldalso allow the optional capacitors Cd to be much smaller than otherwise,or to be omitted entirely. Accordingly, a simple negative voltagegenerator could be used to generate a negative gate voltage bias forjust the end-cap M₀ FETs.

FIG. 6 is a schematic diagram of a series-shunt RF switch circuit 600using the improved positive logic FET stacks of FIG. 4. In theillustrated example, an RF signal from an antenna ANT is coupled fromRF_(IN) through a series stack 602 to RF_(OUT). A shunt stack 604 iscoupled between RF_(OUT) and a reference potential (circuit ground inthis case). Each of the stacks 602, 604 are similar to the FET stack 400of FIG. 4, although “n” may differ for the two stacks 602, 604. (Toavoid clutter, labels for the gate resistors Rg and the resistors Rds ofthe network coupled to the sources or drains of each of the FET M1-Mn,M₀ are omitted). As described above with respect to FIG. 4, each of thestacks 602, 604 is configured to be coupled to two non-negative supplyvoltages, Vgate and Vdrain. Optional capacitors Cd are shown as coupledin parallel with the end-cap M₀ FETs. Notably absent are constantlyconnected DC blocking capacitors or shunt tuning and ESD protectioninductors L, as in FIG. 3.

In a variation of the series-shunt RF switch circuit 600 of FIG. 6, onlyone end of the series stack 602 and/or the shunt stack 604 includes anend-cap M₀ FET.

Stacked FET Switch Bias Ladders

In the embodiments of the invention disclosed above, Vgate is applied toeach FET M1-Mn, M₀ through an associated gate resistor Rg in aconventional biasing scheme, which provides excellent performance inmany applications, including low power RF switch circuits and manyseries and/or shunt circuit configurations. However, for high RF voltageapplications, such as RF tuning switches, an innovative biasing schemeprovides for an enhanced ability to withstand such voltages.

To better understand the problem posed by application of high RFvoltages to a FET stack, FIG. 7 depicts a schematic equivalent circuitof a portion of a prior art FET stack 700, showing parasiticcapacitances. The “top” FET, Mtop, of the FET stack 700 is coupled to anRF input voltage V_(RF), and to a next sequential FET, Mnext. As shouldbe appreciated, additional FETs may be similarly sequentially coupled toMnext.

A conventional parallel gate bias resistor ladder 702 is shown in whicha variable DC gate bias voltage Vgate is coupled to the gate of each FETMtop, Mnext through a corresponding gate resistor Rg. Because the FETsand gate resistors are integrated devices formed on an IC substrate,there are inherent associated parasitic capacitances. In particular,each gate resistor Rg has a parallel parasitic capacitance Cp_(R), andeach FET includes parasitic capacitances Cp_(M) between its gate anddrain and between its gate and source. Accordingly, the gates of eachFET are imperfectly isolated from the gate and source with respect to RFsignals.

Using the example of FIG. 7, if V_(RF) is an RF voltage of 100V and thestack 700 is in the OFF state (i.e., all FETs are biased to blockdrain-source current flow), and if each FET Mtop, Mnext, etc. can safelywithstand 3V from drain to source (further assuming that theresufficient other FETs to drop the applied RF voltage such that no oneFET is subjected to more than 3V), then the source voltage for Mtop (andthus the drain voltage for Mnext) would be 97V, and the source voltagefor Mnext would be 94V; additional FETs in the sequence would similarlydrop 3V from drain to source. The parasitic capacitances Cp_(M) willcause imposition of an RF voltage on the gate of Mtop of about 98.5V(i.e., about one-half the difference between 100V-97V); for Mnext, theimposed gate voltage would be about 95.5V. More specifically, the gateresistor Rg for Mtop will essentially have AC ground (0V) on its Vgateside, and very high RF voltage (−98.5V) on its other side. This causesexcessive power dissipation in the gate resistor Rg for Mtop. Theparallel parasitic capacitor Cp_(R) across that gate resistor Rg makesthe effective resistance even smaller, thereby increasing the RF powerdissipation in the gate resistor Rg, which may cause device failure. Asimilar problem, but at increasingly lower RF voltages, occurs for eachof the successive FETs Mnext in the stack 700.

FIG. 8A is a schematic equivalent circuit of a FET stack 800 with aseries-connected resistor ladder gate bias scheme in accordance with oneaspect of the present invention, showing parasitic capacitances. The“top” FET, Mtop, of the FET stack 800 is coupled to an RF input voltageV_(RF), and to a next sequential FET, Mnext; additional FETs may besimilarly sequentially coupled to Mnext. A variable DC gate bias voltageVgate may be coupled to the gate of each FET Mtop, Mnext at nodes A, Bsituated after a corresponding one of a plurality of series-coupled gateresistors Rg, which thus form a series-connected gate bias resistorladder 802. Stated another way, each gate resistor Rg is coupled to thegates of two adjacent FETs, and the series-connected gate bias resistorladder 802 is configured to be connected to a gate bias voltage Vgate.Accordingly, in the illustrated example, for n FETs, there are n-1 gateresistors Rg (see also FIG. 8B); however, if desired, an additional gateresistor Rg can be coupled between Vgate and the gate of the “bottom”FET in the FET stack 800 (see, for example, resistor R_(BF) in FIG. 8B).Each FET gate will see an applied DC bias voltage of Vgate through theseries connection of the gate resistors Rg of the resistor ladder 802;that is, Vgate essentially just charges or discharges each FET gatethrough corresponding gate resistors Rg.

Using the example values set forth above, if V_(RF) is an RF voltage of100V and each FET Mtop, Mnext, etc. can safely withstand 3V from drainto source, then the source voltage for Mtop (and thus the drain voltagefor Mnext) would be 97V, and the source voltage for Mnext would be 94V.When the stack 800 is in the OFF state, the parasitic capacitancesCp_(M) will enable imposition of an RF voltage on the gate of Mtop ofabout 98.5V; for Mnext, the imposed gate voltage would be about 95.5V.Importantly, because of the changed configuration of theseries-connected gate resistors Rg of the resistor ladder 802, when thestack 800 is in the OFF state, each gate resistor Rg and itscorresponding parasitic capacitance Cp_(R) will be subjected to an RFvoltage of only 3V, representing the difference between the RF voltagesimposed on the gates of adjacent FETs (e.g., 98.5V for the gate of Mtop,and 95.5 for the gate of Mnext). Accordingly, in these examples, whilethe topmost gate resistor for the FET stack 700 of FIG. 7 is subjectedto an RF voltage of 98.5V across its terminals, all gate resistors ofthe FET stack 800 of FIG. 8 are subjected to an RF voltage of only 3V,and thus are protected from breakdown due to excessive RF voltagescapacitively coupled through to the gates of associated FETs.

FIG. 8B is a schematic diagram of a positive logic FET stack 820 with aseries-connected gate bias resistor ladder 802 and a series-connecteddrain-source resistor ladder in accordance with the present invention.The illustrated example is similar to the FET stack 400 of FIG. 4 (withoptional capacitors Cd), but the gate resistors Rg are arrayed in aseries-connected resistor ladder configuration 802 as in FIG. 8A, and,in addition, the resistors Rds of the drain-source resistor network arealso arrayed as a series-connected resistor ladder 804 configured to beconnected to a drain-source bias voltage Vdrain. More specifically, eachresistor Rds is connected across the drain and source of a correspondinginterior FET M1-Mn (but not across the end-cap M₀ FETs, which woulddefeat the DC blocking state of the end-cap M₀ FETs). As is the casewith the series-connected gate bias resistor ladder 802, theseries-connected drain-source resistor ladder 804 prevents any of theRds resistors and FET devices from being subjected to high RF voltagesV_(RF). Note that DC blocking capacitors (not shown) may be coupled toVgate and/or Vdrain as described below with respect to FIG. 8C.

In the example shown in FIG. 8B, the bias voltage Vgate is shown coupledto the “bottom” FET in the FET stack 820 at a “bottom feed” node BFthrough a “bottom feed” resistor R_(BF) at one end of the series ofresistors comprising the gate bias resistor ladder 802. However,alternative connection points may be used if desired; for example, Vgatemay be coupled to the series-connected gate bias resistor ladder 802 ata “middle feed” node MF, optionally through a “middle feed” resistorR_(MF). As should be clear, the “middle feed” node MF need not be in theexact middle of the gate bias resistor ladder 802, but may be anywherebetween the ends of the series of resistors comprising the gate biasresistor ladder 802. A “middle feed” configuration may be useful whenthe FET stack 820 could experience an applied RF voltage to FET at the“top” or “bottom” (in relation to FIG. 8B) of the stack 820, as may bethe case if the FET stack 820 is designed for a dual-polarity seriesapplication (i.e., an RF signal can be applied to either end of thestack) rather than a shunt configuration (see also FIG. 8F below). Inparticular, a “middle feed” configuration can provide a reduction inpower dissipation in the gate resistors Rg regardless of whether an RFvoltage amplitude is applied at either end of the transistor stack.Further benefits of a “middle feed” configuration are described in U.S.patent application Ser. No. 15/289,768 referenced above.

In an optional configuration, an additional parallel-connected gateresistor ladder 805 may be included. The parallel-connected gateresistor ladder 805 comprises respective resistors Rg′ (indicated by ablock symbol) coupled to the gates of the FETs in the FET stack 820. Insome embodiments, a combination of a series-connected gate bias resistorladder 802 and a parallel-connected gate resistor ladder 805 can providereduced RF power dissipation while maintaining a low switching timeconstant (and therefore a high switching speed). Further benefits of a“series plus parallel” gate bias ladder configuration are described inU.S. patent application Ser. No. 15/289,768 referenced above.

FIG. 8C is a schematic diagram of a positive logic FET stack 830 with anadded series-connected resistor ladder for controlling body charge, andthus “back gate” characteristics in the FETs. In this example, each FET(of any type M1-Mn, M₀) is treated as an explicit four-terminaltransistor, and the body of each of the stacked four terminal FETs isconfigured to be connected to a body bias voltage Vbody directly orthrough a corresponding resistor Rb of a body charge control resistorladder 806. As is the case with the series-connected gate bias resistorladder 802 of FIG. 8B, the series-connected body charge control resistorladder 806 prevents any of the resistors Rb from being subjected to highRF voltages V_(RF). A series-connected body charge control resistorladder 806 is also useful for FET stacks that draw a lot of bodycurrent, to mitigate or eliminate interference with gate voltages whenhigh peak RF voltages are applied across the FET stack.

In an optional configuration, an additional parallel-connected gateresistor ladder (not shown, but essentially the same as the parallelgate resistor ladder 805 of FIG. 8C) may be included.

In the circuit configuration of FIG. 8C, a separate Vbody terminal isshown in the schematic diagram for flexibility, but in many applicationsVbody can be tied directly to circuit ground. When Vbody is tieddirectly to circuit ground, all body current in the FETs M1-Mn, M₀ issourced from circuit ground (e.g., V_(GS)=Vbody=0V). In contrast, in aconventional FET stack of the type shown in FIG. 2, body current needsto be sourced from a negative power supply (e.g., V_(GS)=Vbody=−3V).Such a negative power supply is usually provided by a negative chargepump, the output of which, V_(SS), will slump when high peak RF voltagesare applied across the FET stack and induce significant body current.

In some embodiments, capacitors Cgate, Cdrain, and/or Cbody may becoupled as shown to the Vgate, Vdrain, and Vbody terminals,respectively, and to a reference potential, such as circuit ground. TheCgate, Cdrain, and/or Cbody capacitors, when present, serve to filterout RF voltage from DC bias voltages applied to those terminals (i.e.,making Vgate, Vdrain, and/or Vbody AC grounds).

The series-connected gate bias resistor ladder 802 configuration of FIG.8A, the series-connected drain-source resistor ladder 804 of FIG. 8B,and the series-connected body charge control resistor ladder 806 of FIG.8C are applicable to conventional FET stacks (e.g., of the types shownin FIG. 2 or 3), as well as to FET switch stacks having one or moreend-cap M₀ FETs (i.e., FETs that turn OFF when their V_(GS) isessentially zero volts). For example, FIG. 8D is a schematic diagram ofa conventional FET stack 840 with a series-connected gate bias resistorladder 802, a series-connected drain-source resistor ladder 804, and aseries-connected body charge control resistor ladder 806. In contrast tothe embodiment of FIG. 8C, the “end-cap” FETs M1 and Mn are conventionalFETS, additional drain-source resistors Rds′ have been added to thedrain-source resistor ladder 804 in order to span the drain and sourceof each of M1 and Mn, the optional capacitors Cd from FIG. 8C have beenremoved, and there is no Vdrain bias voltage. This configuration has theadvantages for each series-connected resistor ladder 802, 804, 806 notedabove for the series-connected gate bias resistor ladder 802, especiallyprotection from breakdown due to excessive capacitively coupled RFvoltages.

In some embodiments, the series-connected body charge control resistorladder 806 may be replaced by a parallel-connected body charge controlresistor ladder, either in a conventional FET stack or in a FET stackhaving one or more end-cap M₀ FETs. For example, FIG. 8E is a schematicdiagram of a conventional FET stack 850 with a series-connected gatebias resistor ladder 802, a series-connected drain-source resistorladder 804, and a parallel-connected body charge control resistor ladder808. This parallel-connected approach may prove to be advantageous ifthe body current for a series-connected resistor ladder configuration(as in FIG. 8D) would be high enough that the cumulative IR drop wouldcause the voltage on the body of the “top” FET Mn (i.e., the FETfurthest from the input node for the bias voltage Vbody) to becomeundesirably high. The parallel-connected body charge control resistorladder 808 configuration creates a consistent voltage drop across thebody resistors Rb such that there is no additive voltage increase.

In an optional configuration, an additional parallel-connected gateresistor ladder (not shown, but essentially the same as theparallel-connected gate resistor ladder 805 of FIG. 8C) may be included.In some embodiments, the configuration of the body charge controlresistor ladder may be series-connected (as in FIG. 8D) and theconfiguration of the gate bias resistor ladder may be parallel-connected(as in FIG. 4) or a combination of series-connected andparallel-connected (as in FIG. 8B, with the added parallel-connectedgate resistor ladder 805), with benefits similar to those describedabove for FIG. 8E.

As should be appreciated, any combination of series-connected resistorladders 802, 804, 806 may be used in conjunction with a FET stack, withor without adding a parallel-connected gate resistor ladder 805 orsubstituting a parallel-connected body charge control resistor ladder808 for a series-connected resistor ladder 806. Examples include: a gatebias resistor ladder plus a body charge control resistor ladder; a gatebias resistor ladder plus a drain-source resistor ladder; a body chargecontrol resistor ladder plus a drain-source resistor ladder; and a gatebias resistor ladder plus a body charge control resistor ladder plus adrain-source resistor ladder.

If more than one resistor ladder is used in a FET stack that includesend-cap M₀ FETs, it may be necessary or useful to provide anon-overlapping sequencing for states changes to the Vgate, Vdrain, andVbody voltages to help prevent “shoot through” DC current in the end-capM₀ FETs. Such current may occur because the positive-logic FETs (e.g.,FETs M1-Mn in FIG. 4) have different ON-to-OFF and OFF-to-ON switchingcharacteristics and timing compared to the essentially zero V_(GS) M₀FETs. For example, referring to FIG. 8C, when the FET stack is beingswitched from the OFF state to the ON state, it may be preferable tochange Vdrain (e.g., from 3V to 0V) before changing Vgate (e.g., from 0Vto 3V). Conversely, when the FET stack is being switched from the ONstate to the OFF state, it may be preferable to change Vgate (e.g., from3V to 0V) before changing Vdrain (e.g., from 0V to 3V).

Bias Ladders with AC Coupling Modules

The series-connected or series/parallel connected gate resistor laddersdescribed above allow the gate of each FET in a switch stack to trackthe common mode voltage of its associated FET. In general, such gateresistor ladders must provide a high impedance relative to the impedance(capacitance) between the gate and the source/drain of the FETs in theswitch stack. Despite the benefits of a series-connected gate biasresistor ladder 802 (e.g., as shown in FIG. 8A) or of a combination ofseries-connected gate bias resistor ladder 802 and a parallel-connectedgate bias resistor ladder 805 (e.g., as shown in FIG. 8B) as describedto this point, in some applications, the FET nearest an applied RFsignal (e.g., the “top” FET in FIGS. 8B-8E coupled to V_(RF)) still hasan effective RF load that can cause the gate of that FET to not fullytrack the transistor common mode voltage during RF operation. The RFload on the gate of the “top” FET acts as parasitic capacitance inproducing non-ideal voltage division, and if not compensated, can reducethe peak breakdown voltage (BV) withstand capability of that FET byseveral percent.

However, the load seen by that “top” FET can be substantially mitigatedor even totally eliminated by coupling an AC Coupling Gate Module 822between an RF input terminal and the gate of the FET (through at least agate resistor) in the stack nearest the RF input terminal. For example,in FIGS. 8B-8E, an AC Coupling Gate Module 822 is coupled between theterminal receiving a V_(RF) signal and the gate of the “top” FET (Mn orM₀, as the case may be)—thus, the AC Coupling Gate Module 822 is alsocoupled to the “top” of the series-connected gate bias resistor ladder802.

As another example, FIG. 8F is a schematic diagram of a conventional FETstack 860 with a series-connected gate bias resistor ladder 802 and anAC Coupling Gate Module 822. The FET stack 860 is configured to becoupled in a shunt configuration or in a series configuration. In ashunt configuration, the RF− terminal is usually circuit ground, an RFinput signal is applied at the RF+ terminal, and Vgate is applied at the“bottom” of the FET stack 860 through resistor R_(BF). In a seriesconfiguration, either or both of the RF+ and RF− terminals may be thesource of an RF signal (e.g., RF+ may be coupled between an RF antennaand an amplifier), and Vgate would generally be applied at the “middle”of the FET stack 860 through resistor R_(MF). As noted above, a “middlefeed” configuration can provide a reduction in power dissipation in thegate resistors Rg regardless of whether an RF voltage amplitude isapplied at either end of the FET stack 860. As should be clear, the FETsM1-Mn of the conventional FET stack 860 of FIG. 8F can be replaced by apositive logic stack of the type shown in FIG. 8B.

In some embodiments, the AC Coupling Gate Module 822 may comprise a DCblocking capacitor C 1, while in other embodiments the AC Coupling GateModule 822 may comprise a DC blocking capacitor C1 connected in serieswith a resistor R1 (the order of connection of C1 and R1 does notmatter, whereas the order of connection does matter for Cgate and R_(BF)at the “bottom” of the stack for a shunt stack configuration). The ACCoupling Gate Module 822 improves voltage division along the gate biasresistor ladder 802 and allows the gate of the “top” FET to more fullytrack the transistor common mode voltage during RF operation. Anotherbenefit of the AC Coupling Gate Module 822 in a series configuration isthat it lets a coupled antenna be loaded directly, rather than throughthe FETs of the stack.

As noted above, in a series configuration, either or both of the RF+ andRF− terminals may be the source of an RF signal, and Vgate wouldgenerally be applied at the “middle” of the FET stack 860 throughresistor R_(MF). In such a case, it may be beneficial to couple aCoupling Gate Module between the series-connected gate bias resistorladder 802 and both of the RF+ and RF− terminals. For example, FIG. 8Gis a schematic diagram of a conventional FET stack 870 with aseries-connected gate bias resistor ladder 802 and dual AC Coupling GateModules. The FET stack 870 of FIG. 8G is similar to the FET stack 860 ofFIG. 8F, but with a “middle feed” configuration for Vgate, a first ACCoupling Gate Module 822 at the “top” of the stack nearest FET Mn, and asecond AC Coupling Gate Module 822′ at the “bottom” of the stack nearestFET M1. The second AC Coupling Gate Module 822′ may comprise a DCblocking capacitor C2, while in other embodiments the second AC CouplingGate Module 822′ may comprise a DC blocking capacitor C2 connected inseries with a resistor R2. With the illustrated configuration of ACCoupling Gate Modules 822, 822′, the load seen by the FETs nearest theRF+ and RF− terminals can be substantially mitigated or even totallyeliminated.

Referring back to FIG. 8F, if a series-connected body charge controlresistor ladder 806 is included in the FET stack 860, a similar ACCoupling Body Module 823 will provide the same benefits for the body (or“back gate”) of the “top” FET (in this example, FET Mn, nearest the RF+terminal). In the illustrated embodiment, the AC Coupling Body Module823 is coupled between the RF+ terminal and the series-connected bodycharge control resistor ladder 806 connected to the “top” FET Mn nearestthe RF+ terminal. The AC Coupling Body Module 823 may comprise a DCblocking capacitor C2 in some embodiments, while in other embodimentsthe AC Coupling Body Module 823 may comprise a DC blocking capacitor C2connected in series with a resistor R2 (again, the order of connectiondoes not matter). Further, in a series configuration, a second ACCoupling Body Module (not shown) may be coupled between the RF− terminaland the series-connected body charge control resistor ladder 806connected to the “bottom” FET M1 nearest the RF− terminal to providesimilar benefits.

An AC Coupling Gate Module and an AC Coupling Body Module may be usedindependently of each other, or may be used together. In general, whenusing a series-connected gate bias resistor ladder 802 and/or aseries-connected body charge control resistor ladder 806, there aresignificant benefits to using an AC Coupling Gate and/or Body Modulebetween the respective ladders 802, 806 and any RF terminal presentingan RF load, in order to mitigate or even totally eliminate an RF loadacross the FET coupled to that RF terminal.

In some embodiments, a parallel-connected gate bias ladder 805 may beincluded with the values of the resistors Rg′ being selected to isolatefor a different voltage division in the gate bias resistor laddersversus the component FETs M1-Mn of the FET stack 860. More particularly,if the voltage division across the FET stack 860 is uniform, thenconstant Rg values for a series-connected gate bias resistor ladder 802(i.e., a “rail” only configuration) in conjunction with an AC CouplingGate Module 822 completely eliminates the RF load on the FETs since thevoltage division across the gate bias resistor ladder is uniform. On theother hand, if there is non-ideal voltage division across the FET stack860, a mismatch occurs between the voltage division across the FETs andthe voltage division across the rail-only gate bias resistor ladder.This can be remedied by including a parallel-connected gate bias ladder805 (i.e., adding “rungs” to the “rails” of the series-connected gatebias resistor ladder 802). Adding rung resistors Rg′ helps reduce the RFload due to non-ideal voltage division across the FET stack 860, withminimal switching time constant penalty.

In some gate bias ladder embodiments having a series-connected gate biasresistor ladder 802 (a “rail” configuration) and a parallel-connectedgate bias ladder 805 (a “rung” configuration), there is a benefit tohaving variable values for the resistors of either or both of the “rail”and “rung” bias resistor ladders 802, 805 (as opposed to constantresistance values). In general, the variation should be tapered from thetop of the FET stack to the bottom; that is, the resistor values in oneor both of the bias resistor ladders 802, 805 should be higher near thetop, tapering to lower at the bottom. For example, compared to constantvalue rail and rung resistors, there is significant improvement in peakbreakdown voltage (BV) for the top-most transistor (nearest the RFsignal input) when the rail resistors are variable and the rungresistors are constant, while retaining similar switching time and powerdissipation characteristics. In addition, a “variable rail/constantrung” gate bias resistor ladder configuration allows the size of therung resistors to be much smaller than with other configurations,enabling greater flexibility in designing IC layouts. Similar benefitsmay be seen in embodiments using a “variable rail/variable rung” gatebias resistor ladder configuration. Note that variable rail resistorvalues may be adjusted to achieve a desired relationship of (1) the RFload on the FET gates to (2) the peak BV for the top-most transistor.However, there is a tradeoff with the switching time constant.

Note that when the FET stack 860 is coupled to an antenna or the like,then the resistance of R1 is preferably set to one-half of theresistance of the series-connected resistors Rg (i.e., R1=½ Rg), sincethe gate of the “top” FET sees only one-half the voltage below theantenna compared to the voltage across each other FET.

Notably, the inclusion of the AC Coupling Gate Module 822 and/or the ACCoupling Body Module 823 has essentially no adverse effect on theperformance characteristics of the FET stack 860 (e.g., in terms ofswitching time constant, resistor power dissipation, resistive voltagedrop through body resistors due to body current, etc.). On the otherhand, the inclusion of one or more of such modules 822, 823substantially eliminates the reduction in the peak breakdown voltage(BV) withstand capability of the “top” FET of the FET stack that wouldotherwise occur. As one specific example, using modeled circuits tocompare (1) a FET stack configuration using a series-connected gate biasresistor ladder 802 but no AC Coupling Gate Module 822 against (2) a FETstack configuration using a series-connected gate bias resistor ladder802 and an AC Coupling Gate Module 822, the former configuration showeda decrease in BV withstand capability of the “top” FET of about 0.714,while the latter configuration showed a decrease in BV withstandcapability of the “top” FET of about 0.001—a factor of over 700 inimprovement due to the AC Coupling Gate Module 822.

Note that in a series configuration for the FET stack 860, where eitheror both of the RF+ and RF− terminals may be the source of an RF signal,an AC Coupling Gate Module 822 may be coupled between a terminal RF+,RF− and a respective end of the series-connected gate bias resistorladder 802 for the FET stack 860. In such a case, Vgate would normallybe applied to the FET stack 860 using a “middle feed” configuration.Similarly, an AC Coupling Body Module 823 may be coupled to each end ofthe body charge control resistor ladder 806 for the FET stack 860.

Accumulated Charge Sink Structure

Embodiments of the invention, including the embodiments described above,that are fabricated using silicon-on-insulator (SOI)—includingsilicon-on-sapphire (SOS) MOSFETs—may benefit by adding an accumulatedcharge sink (ACS) structure to one or more constituent FETs (i.e., M1-Mnand/or M₀). In particular, the linearity of an SOI MOSFET can beimproved by providing a means for sweeping out accumulated chargetrapped below the gate oxide of the MOSFET, such as by use of the“HaRP”™ accumulated charge sink (ACS) technology taught in U.S. Pat. No.7,910,993, issued Mar. 22, 2011, entitled “Method and Apparatus for usein Improving Linearity of MOSFETs Using an Accumulated Charge Sink”,which is hereby incorporated by reference. An “accumulated charge”, asdefined in more detail in U.S. Pat. No. 7,910,993 and as used herein,refers to gate-bias induced carriers that may accumulate in the body ofan OFF-state MOSFET, even if the majority carriers in the body do nothave the same polarity as the accumulated charge.

More particularly, as described in greater detail in U.S. Pat. No.7,910,993, no matter what mode of operation an SOI MOSFET employs (i.e.,whether enhancement or depletion mode), when the MOSFET is operated inan OFF-state (i.e., the gate voltage does not exceed V_(th)), and when asufficient nonzero gate bias voltage is applied with respect to theMOSFET source and drain, an “accumulated charge” may occur under theMOSFET gate which adversely affects the linearity and other aspects ofthe performance of the MOSFET. To alleviate the effects of suchaccumulated charge, an SOI MOSFET may include an ACS structure thatallows discharge of accumulated charge when the MOSFET is in anon-conducting (OFF) mode while allowing normal operation when theMOSFET is in a conducting (ON) mode.

FIG. 8H is a simplified schematic diagram of an SOI MOSFET 810 thatincludes an ACS structure along with conventional drain D, source S, andgate G terminals and a gate resistor Rg. In this particular illustratedexample, the ACS structure comprises a diode 812 coupled from the bodyof the MOSFET 810 to the gate G of the MOSFET 810. The diode 812discharges accumulated charge from the body of the MOSFET 810 throughthe gate G when the MOSFET 810 is in a non-conducting (OFF) mode, whileallowing normal operation when the MOSFET 810 is in a conducting (ON)mode. Alternative ACS structure circuits and methods for reducingaccumulated charge in floating body semiconductor devices are describedin greater detail in U.S. Pat. No. 7,910,993 referenced above. TheMOSFET 810 along with an ACS structure (e.g., a body-to-gate diode 812)may also be referred to as an “ACS FET”. As should be clear, in an SOIMOSFET stack comprising one or more series-coupled positive-logicMOSFETs M1-Mn (i.e., FETs not requiring a negative voltage supply toturn OFF), coupled to one or more series-coupled “end-cap” M₀ MOSFETs,one or more of the MOSFETs may be ACS FETs.

Methods

Note that in specifying a particular circuit embodiment having thearchitecture and characteristics of the example circuits describedabove, the principal of “superposition” may be used as a method foroptimizing DC biasing and best AC voltage handling configurationindependently, and then combining the results. That is, one can firstdefine an optimum circuit configuration independently for AC behaviorand DC behavior, and then overlay and merge the two solutions.Mathematically, superposition may be expressed as f(DC+AC)=f(DC)+f(AC),where “DC” is the DC behavior of the circuit, and “AC” is the ACbehavior of the circuit.

Another aspect of the invention includes methods for providing a FETswitch stack. For example, FIG. 9 is a process flow diagram 900 showingone method for providing a FET switch stack. The method includes:providing one or more series-coupled positive-logic FETs (STEP 902); andproviding a first end-cap FET of a type that turns OFF when the V_(GS)of such end-cap FET is essentially zero volts, series-coupled to a firstend of the one or more series-coupled positive-logic FETs (STEP 904).

Additional aspects of the method of FIG. 9 may include one or more ofthe following: providing a second end-cap FET of a type that turns OFFwhen the V_(GS) of such end-cap FET is essentially zero volts,series-coupled to a second end of the one or more series-coupledpositive-logic FETs; further including series coupling, to the FETswitch stack, at least one additional FET of the type that turns OFFwhen the V_(GS) of such FET is essentially zero volts; wherein eachend-cap FET functions as a DC blocking capacitor when in an OFF state,and as a resistive signal path when in an ON state; further includingcoupling at least one capacitor between the source and the drain of acorresponding end-cap FET; further including providing a gate biasresistor ladder including a plurality of series-coupled resistorsconfigured to be coupled to a gate bias voltage, wherein each resistoris coupled to the respective gates of corresponding adjacent FETs;further including a capacitor coupled between the gate bias resistorladder and a reference potential; further including coupling an ACcoupling gate module to at least one end of the gate bias resistorladder and configuring the AC coupling gate module to be coupled to aradio frequency voltage source; wherein the AC coupling gate moduleincludes one of a capacitor or a capacitor series coupled to a resistor;further including providing a body charge control resistor ladderincluding a plurality of series-coupled or parallel-coupled resistorsconfigured to be coupled to a body bias voltage, wherein each resistoris coupled to the body of at least one corresponding FET; furtherincluding a capacitor coupled between the body charge control resistorladder and a reference potential; further including coupling an ACcoupling body module to at least one end of the body charge controlresistor ladder, the AC coupling body module configured to be coupled toa radio frequency voltage source; wherein the AC coupling body moduleincludes one of a capacitor or a capacitor series coupled to a resistor;further including providing a drain-source resistor ladder including aplurality of series-coupled resistors configured to be coupled to adrain-source bias voltage, wherein each resistor is coupled to therespective drains and sources of corresponding adjacent positive-logicFETs; further including coupling a negative gate bias voltage supply tothe gates of the end-cap FETs; and wherein at least one FET is an ACSFET.

As another method example, FIG. 10 is a process flow diagram 1000showing one method for providing a series-shunt switch circuit. Themethod includes: providing a series FET switch stack having an inputport configured to be coupled in series with an RF signal line, and anoutput port (STEP 1002); coupling a shunt FET switch stack to the outputport of the series FET switch stack (STEP 1004); configuring the shuntFET switch stack to be coupled to a reference potential (STEP 1006);wherein the series FET switch stack and the shunt FET switch stack haveat least one end-cap FET that turns OFF when the V_(GS) of such end-capFET is essentially zero volts (STEP 1008), or wherein the series FETswitch stack and the shunt FET switch stack include at least oneseries-coupled positive-logic FET, series-coupled to at least oneend-cap FET of a type that turns OFF when the V_(GS) of such end-cap FETis essentially zero volts (STEP 1010).

As yet another method example, FIG. 11 is a process flow diagram 1100showing one method for biasing a FET switch stack. The method includes:providing a plurality of series-coupled FETs (STEP 1102); providing agate bias resistor ladder including a plurality of resistors configuredto be coupled to a gate bias voltage and to the gates of at least onecorresponding FET (STEP 1104); providing a body charge control resistorladder including a plurality of resistors configured to be coupled to abody bias voltage and to the body of at least one corresponding FET(STEP 1106); and providing a drain-source resistor ladder including aplurality of series-coupled resistors configured to be coupled to adrain-source bias voltage, wherein each resistor is coupled to therespective drains and sources of at least one corresponding adjacent FET(STEP 1108); and wherein the resistors of the gate bias resistor ladderare series-connected and the resistors of the body charge controlresistor ladder are parallel-connected (STEP 1110), or alternatively,wherein the resistors of the gate bias resistor ladder areparallel-connected and the resistors of the body charge control resistorladder are series-connected (STEP 1112).

Additional aspects of the method of FIG. 11 may include one or more ofthe following: wherein the plurality of series-coupled FETs includes oneor more series-coupled positive-logic FET, series-coupled on at leastone end to an end-cap FET of a type that turns OFF when the VGS of suchend-cap FET is essentially zero volts; further including a secondend-cap FET of a type that turns OFF when the V_(GS) of such end-cap FETis essentially zero volts, series-coupled to a second end of the one ormore series-coupled positive-logic FETs; coupling a capacitor betweenthe gate bias resistor ladder and a reference potential; coupling an ACcoupling gate module to at least one end of the gate bias resistorladder and configuring the AC coupling gate module to be coupled to aradio frequency voltage source; wherein the AC coupling gate moduleincludes one of a capacitor or a capacitor series coupled to a resistor;further including coupling an AC coupling body module coupled to atleast one end of the body charge control resistor ladder and configuringthe AC coupling gate module to be coupled to a radio frequency voltagesource; wherein the AC coupling body module includes one of a capacitoror a capacitor series coupled to a resistor; coupling a capacitorbetween the body charge control resistor ladder and a referencepotential; configuring the gate bias resistor ladder to be coupled to abias voltage at a node at a first end of the gate bias resistor ladder;configuring the gate bias resistor ladder to be coupled to a biasvoltage at a node between a first end and a second end of the gate biasresistor ladder; wherein at least one FET is an ACS FET; furtherincluding coupling a parallel-connected gate resistor ladder, includinga plurality of resistors, between the gate bias resistor ladder and thegates of corresponding FETs; wherein the plurality of series-coupledFETs includes at least one series-coupled positive-logic FET, bracketedby series-coupled end-cap FETs of a type that turn OFF when the V_(GS)of such end-cap FETs is essentially zero volts; and wherein theplurality of series-coupled FETs includes a first end-cap FET of a typethat turns OFF when the V_(GS) of such end-cap FET is essentially zerovolt, a second end-cap FET of a type that turns OFF when the V_(GS) ofsuch end-cap FET is essentially zero volts, and one or moreseries-coupled positive-logic FETs, series coupled between the firstend-cap FET and the second end-cap FET.

As still another method example, FIG. 12 is a process flow diagram 1200showing a method for providing a FET switch stack. The method includes:providing a plurality of series-coupled FETs (STEP 1202); coupling agate bias resistor ladder to the gates of the series-coupled FETs (STEP1204); coupling an AC coupling gate module to at least one end of thegate bias resistor ladder and configuring the AC coupling gate module tobe coupled to a corresponding radio frequency voltage source (STEP1206); optionally, coupling a body charge control resistor ladder to thebodies of the series-coupled FETs (STEP 1208); and, optionally, couplingan AC coupling body module to at least one end of the body chargecontrol resistor ladder and configured to be coupled to thecorresponding radio frequency voltage source (STEP 1210).

Additional aspects of the method of FIG. 12 may include one or more ofthe following: wherein the plurality of series-coupled FETs includes oneor more series-coupled positive-logic FET, series-coupled on at leastone end to an end-cap FET of a type that turns OFF when the V_(GS) ofsuch end-cap FET is essentially zero volts; further including providinga second end-cap FET of a type that turns OFF when the V_(GS) of suchend-cap FET is essentially zero volts, series-coupled to a second end ofthe one or more series-coupled positive-logic FETs; further includingproviding a drain-source resistor ladder including a plurality ofseries-coupled resistors configured to be coupled to a drain-source biasvoltage, wherein each resistor is coupled to the respective drains andsources of corresponding adjacent positive-logic FETs; wherein the gatebias resistor ladder includes a plurality of series-connected resistors;wherein the body charge control resistor ladder includes a plurality ofseries-connected resistors; wherein the AC coupling gate module includesone of a capacitor or a capacitor series coupled to a resistor; whereinthe AC coupling body module includes one of a capacitor or a capacitorseries coupled to a resistor; wherein the gate bias resistor ladder isconfigured to be coupled to a bias voltage at a node at a first end ofthe gate bias resistor ladder; wherein the gate bias resistor ladder isconfigured to be coupled to a bias voltage at a node between a first endand a second end of the gate bias resistor ladder; and wherein at leastone FET is an ACS FET.

As yet another method example, FIG. 13 is a process flow diagram 1300showing another method for biasing a FET switch stack. The methodincludes: providing a plurality of series-coupled FETs (STEP 1302);providing a first gate resistor ladder including a plurality ofparallel-connected constant-valued (or, optionally, variable-valued)resistors each coupled to the gate of one corresponding FET (STEP 1304);and providing a second gate bias resistor ladder including a pluralityof series-connected variable-valued resistors each coupled to onecorresponding constant-valued resistor of the first gate resistor ladder(STEP 1306).

Fabrication Technologies & Options

The term “MOSFET”, as used in this disclosure, means any field effecttransistor (FET) with an insulated gate and comprising a metal ormetal-like, insulator, and semiconductor structure. The terms “metal” or“metal-like” include at least one electrically conductive material (suchas aluminum, copper, or other metal, or highly doped polysilicon,graphene, or other electrical conductor), “insulator” includes at leastone insulating material (such as silicon oxide or other dielectricmaterial), and “semiconductor” includes at least one semiconductormaterial.

As should be readily apparent to one of ordinary skill in the art,various embodiments of the invention can be implemented to meet a widevariety of specifications. Unless otherwise noted above, selection ofsuitable component values is a matter of design choice and variousembodiments of the invention may be implemented in any suitable ICtechnology (including but not limited to MOSFET structures), or inhybrid or discrete circuit forms. Integrated circuit embodiments may befabricated using any suitable substrates and processes, including butnot limited to standard bulk silicon, silicon-on-insulator (SOI), andsilicon-on-sapphire (SOS). Unless otherwise noted above, the inventionmay be implemented in other transistor technologies such as bipolar,GaAs HBT, GaN HEMT, GaAs pHEMT, and MESFET technologies. However, theinventive concepts described above are particularly useful with anSOI-based fabrication process (including SOS), and with fabricationprocesses having similar characteristics. Fabrication in CMOS on SOI orSOS processes enables circuits with low power consumption, the abilityto withstand high power signals during operation due to FET stacking,good linearity, and high frequency operation (i.e., radio frequencies upto and exceeding 50 GHz). Monolithic IC implementation is particularlyuseful since parasitic capacitances generally can be kept low (or at aminimum, kept uniform across all units, permitting them to becompensated) by careful design.

Voltage levels may be adjusted, or voltage and/or logic signalpolarities reversed, depending on a particular specification and/orimplementing technology (e.g., NMOS, PMOS, or CMOS, and enhancement modeor depletion mode transistor devices). Component voltage, current, andpower handling capabilities may be adapted as needed, for example, byadjusting device sizes, serially “stacking” components to withstandgreater voltages as described above, and/or using multiple components inparallel to handle greater currents. Additional circuit components maybe added to enhance the capabilities of the disclosed circuits and/or toprovide additional functional without significantly altering thefunctionality of the disclosed circuits.

Conclusion

A number of embodiments of the invention have been described. It is tobe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, some of thesteps described above may be order independent, and thus can beperformed in an order different from that described. Further, some ofthe steps described above may be optional. Various activities describedwith respect to the methods identified above can be executed inrepetitive, serial, or parallel fashion.

It is to be understood that the foregoing description is intended toillustrate and not to limit the scope of the invention, which is definedby the scope of the following claims, and that other embodiments arewithin the scope of the claims. (Note that the parenthetical labels forclaim elements are for ease of referring to such elements, and do not inthemselves indicate a particular required ordering or enumeration ofelements; further, such labels may be reused in dependent claims asreferences to additional elements without being regarded as starting aconflicting labeling sequence).

1. A FET switch stack, including: (a) a plurality of series-coupledFETs; (b) a gate bias resistor ladder including a plurality of resistorsconfigured to be coupled to a gate bias voltage and to the gate of atleast one corresponding FET; (c) a body charge control resistor ladderincluding a plurality of resistors configured to be coupled to a bodybias voltage and to the body of at least one corresponding FET; and (d)a drain-source resistor ladder including a plurality of series-coupledresistors configured to be coupled to a drain-source bias voltage,wherein each resistor is coupled to the respective drains and sources ofat least one corresponding adjacent FET; wherein the resistors of thegate bias resistor ladder are series-connected and the resistors of thebody charge control resistor ladder are parallel-connected; and whereinthe plurality of series-coupled FETs includes one or more series-coupledpositive-logic FETs requiring a relative negative V_(GS) to turn OFF butconfigured to not require a negative power supply, and a first end-capFET that turns OFF when the V_(GS) of such first end-cap FET isessentially zero volts, series-coupled to a first end of the one or moreseries-coupled positive-logic FETs.
 2. A FET switch stack, including:(a) a plurality of series-coupled FETs; (b) a gate bias resistor ladderincluding a plurality of resistors configured to be coupled to a gatebias voltage and to the gate of at least one corresponding FET; (c) abody charge control resistor ladder including a plurality of resistorsconfigured to be coupled to a body bias voltage and to the body of atleast one corresponding FET; and (d) a drain-source resistor ladderincluding a plurality of series-coupled resistors configured to becoupled to a drain-source bias voltage, wherein each resistor is coupledto the respective drains and sources of at least one correspondingadjacent FET; wherein the resistors of the gate bias resistor ladder areparallel-connected and the resistors of the body charge control resistorladder are series-connected; and wherein the plurality of series-coupledFETs includes one or more series-coupled positive-logic FETs requiring arelative negative V_(GS) to turn OFF but configured to not require anegative power supply, and a first end-cap FET that turns OFF when theV_(GS) of such first end-cap FET is essentially zero volts,series-coupled to a first end of the one or more series-coupledpositive-logic FETs.
 3. (canceled)
 4. The invention of claim 1 or 2,further including a second end-cap FET that turns OFF when the VGS ofsuch second end-cap FET is essentially zero volts, series-coupled to asecond end of the one or more series-coupled positive-logic FETs.
 5. Theinvention of claim 1 or 2, further including a capacitor coupled betweenthe gate bias resistor ladder and a reference potential.
 6. Theinvention of claim 1 or 2, further including an AC coupling gate modulecoupled to at least one end of the gate bias resistor ladder andconfigured to be coupled to a radio frequency voltage source. 7.(canceled)
 8. The invention of claim 1 or 2, further including an ACcoupling body module coupled to at least one end of the body chargecontrol resistor ladder and configured to be coupled to a radiofrequency voltage source.
 9. (canceled)
 10. The invention of claim 1 or2, further including a capacitor coupled between the body charge controlresistor ladder and a reference potential.
 11. (canceled)
 12. (canceled)13. The invention of claim 1 or 2, wherein at least one FET is an ACSFET.
 14. The invention of claim 1, further including aparallel-connected gate resistor ladder, including a plurality ofresistors coupled between the gate bias resistor ladder and the gates ofcorresponding FETs.
 15. A FET switch stack, including: (a) a pluralityof series-coupled FETs having a first FET configured to be coupled to aradio frequency (RF) signal input; (b) a first gate resistor ladderincluding a plurality of parallel-connected constant-valued resistorseach coupled to the gate of one corresponding FET; and (c) a second gatebias resistor ladder including a plurality of series-connectedvariable-valued resistors each coupled to one correspondingconstant-valued resistor of the first gate resistor ladder and havingresistive values that taper from higher resistive values to lowerresistive values, with the higher resistive values located near the RFsignal input to the first FET.
 16. A FET switch stack, including: (a) aplurality of series-coupled FETs having a first FET configured to becoupled to a radio frequency (RF) signal input; (b) a first gateresistor ladder including a plurality of parallel-connectedvariable-valued resistors each coupled to the gate of one correspondingFET; and (c) a second gate bias resistor ladder including a plurality ofseries-connected variable-valued resistors each coupled to onecorresponding constant-valued resistor of the first gate resistor ladderand having resistive values that taper from higher resistive values tolower resistive values, with the higher resistive values located nearthe RF signal input to the first FET. 17-32. (canceled)